Thermal or vapor phase cracking of saturated hydrocarbons to prepare unsaturated hydrocarbons has been known since the latter part of this century. Many important features, such as the production of olefin and aromatic hydrocarbons and the use of steam to prevent carbon formation, have been reported.
It has now been discovered that recoverable yields of carboxylic acids are prepared by vapor-phase cracking of high-molecular-weight paraffins under select conditions in the presence of carbon dioxide. In particular, it has been found that paraffins containing at least 12 carbon atoms when cracked at a temperature of at least 500.degree. C and a residence time of from about 0.1 seconds to about 10 seconds in the presence of from about 0.5 to about 5 mols of carbon dioxide per mol of hydrocarbon are converted at recoverable yields to carboxylic acids.
Carbon dioxide has been employed in vapor phase dehydrogenation of saturated and unsaturated hydrocarbons under conditions which promote the dehydrogenation reaction; but has not been employed to produce carboxylic acids. For example, U.S. Pat. No. 2,775,631 granted Dec. 25, 1956 describes vapor phase dehydrogenation of C.sub.5 to C.sub.8 olefins to prepare aromatics in the presence of carbon dioxide and a molybdenum oxide catalyst; U.S. Pat. No. 3,406,219 granted Oct. 15, 1968 describes the dehydrogenation of ethylbenzene with at least 3 mols of carbon dioxide in the presence of a Fischer-Tropsch catalyst to prepare styrene; and U.S. Pat. No. 3,505,422 granted Apr. 7, 1970 describes the dehydrogenation of hydrocarbons by a catalytic process in which minor amounts of carbon dioxide are added to steam to increase the consumption of hydrogen produced.
Carbon dioxide has been employed in the vapor phase conversion of para-xylene to prepare terephthalic acid. For example, Higuchi et al, Kogyo Kagaku Zasshi, 1968, 71(10), 1663-6 describes the thermal decomposition of p-xylene diluted in carbon dioxide at a temperature of 860.degree. C to 1050.degree. C to prepare the aromatic acid.
Carbon dioxide has also been employed in carboxylations effected by ionizing radiation. For example, McKusick et al, Journal of the American Chemical Society, 1960, 82, 723 describe irradiation of a mixture of hydrocarbon and carbon dioxide with high energy electrons to give carboxylic acids.
While carbon dioxide has been employed to promote dehydrogenation by reacting with hydrogen to form water and carbon monoxide, the process of the present invention employs relatively mild vapor phase cracking conditions and a high molecular weight paraffin which are suitable to effect reaction between the paraffin and carbon dioxide to form carboxylic acids. This should not be confused with the formation of aromatic carboxylic acids under extreme conditions of high temperature. At high temperatures, above about 1000.degree. C, carbon dioxide can oxidize the aromatic to its corresponding acid, while being reduced to carbon monoxide. Under conditions of this invention carbon dioxide is not an oxidizing agent.